Thermal Compressive Bonding with Separate Die-Attach and Reflow Processes

ABSTRACT

A method of bonding includes providing a first work piece, and attaching a second work piece on the first work piece, with a solder bump disposed between the first and the second work pieces. The second work piece is heated using a heating head of a heating tool to melt the solder bump. After the step of heating the second work piece, one of the first and the second work pieces is allowed to move freely in a horizontal direction to self-align the first and the second work pieces. After the step of allowing one of the first and the second work pieces to move, a temperature of the heating head is lowed until the first solder bump solidifies to form a second solder bump.

This application is a continuation of U.S. patent application Ser. No.12/874,009, filed Sep. 1, 2010, and entitled “Thermal CompressiveBonding with Separate Die-Attach and Reflow Processes,” whichapplication is hereby incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATION

This application relates to the following commonly-assigned U.S. patentapplication: application Ser. No. 12/841,858, filed Jul., 22, 2010, andentitled “Thermal Compress Bonding,” which application is herebyincorporated herein by reference.

BACKGROUND

Integrated circuits are formed on semiconductor wafers, which are thensawed into semiconductor chips. The semiconductor chips may be bondedonto package substrates. During the bonding process, the solder bumpsbetween the semiconductor chips and the package substrates are reflowed.Conventional reflow methods include convection-type reflow and thermalcompressive reflow. The convection-type reflow has relatively highthroughput since a plurality of package substrates and the overlyingdies may be bonded through the reflow at the same time. However, theconvection-type reflow requires a long period of time to heat solderbumps. The resulting high thermal budget may cause significant warpagein the dies, and may possibly cause delamination between low-kdielectric layers.

The thermal compressive bonding requires a lower thermal budget than theconvection-type reflow. However, the thermal compressive bonding has avery low throughput. During the thermal compressive bonding, a bond headpicks up a die, flips the die, and attaches the die to a packagesubstrate. The bond head then goes through a temperature ramp-up processto heat the die and the solder bumps that join the die and the packagesubstrate. After the solder bumps are melted, the bond head goes througha cool-down process so that the solder bumps solidify. This process isrepeated for each of the dies, and hence the throughput of the thermalcompressive bonding is very low, which sometimes may be only 1/15 of thethroughput of the convection-type reflow.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 and 2 illustrate perspective views of a body and a cover of ajig-type substrate carrier;

FIG. 3 illustrates a die-attach head picking up a die;

FIG. 4 illustrates the process of attaching a die onto a packagesubstrate, wherein the package substrate is located in a work pieceholder in the jig-type substrate carrier;

FIGS. 5A and 5B illustrate a perspective view and a cross-sectional viewin the reflowing of solder bumps between dies and package substrates,wherein a multi-head heating tool is used for heating the solder bumps;

FIG. 6 illustrates that dies are lifted up by the heating tool;

FIGS. 7 through 9 illustrate cross-sectional views of intermediatestages in the reflowing of solder bumps between dies and packagesubstrates in accordance with alternative embodiments;

FIGS. 10 and 11 illustrate the reflowing of the solder bumps using atwo-head heating tool and a four-head heating tool, respectively; and

FIG. 12 illustrates a schematic temperature profile of heating heads inthe multi-head heating tool.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative, and do not limit the scope of the disclosure.

A novel thermal compressive bonding (TCB) process, sometimes known asthermal compression bonding process, is provided. The intermediatestages of various embodiments are illustrated. Variations of theembodiments are also illustrated. Throughout the various views andillustrative embodiments, like reference numbers are used to designatelike elements.

FIGS. 1 and 2 illustrate a body and a cover of jig-type substratecarrier 20. Referring to FIG. 1, body 20A of jig-type substrate carrier20 includes a plurality of work piece holders 22, which may be arrangedas an array having a plurality of rows and a plurality of columns.Although FIG. 1 only illustrates work piece holders 22 as a 2×3 array,substrate carrier 20 may have any number of rows and columns of workpiece holders. Work piece holders 22 may include portions of body 20Athat form the sidewalls and bottoms of holes, in which work pieces maybe placed.

Referring to FIG. 2, cover 20B of jig-type substrate carrier 20 isprovided. Cover 20B also includes a plurality of openings 24. Cover 20Bcan be secured on body 20A so that cover 20B and body 20A can betransported and used as an integrated unit. When cover 20B is secured onbody 20A, each of the openings 24 is directly over one of work pieceholders 22. Further, length L1 and width W1 of body 20A (FIG. 1) aregreater than the respective length L2 and width W2 of cover 20B, so thatafter cover 20B is placed on body 20A, the edge portions of each of workpiece holders 22 are covered by portions of cover 20B, while the centerportion of each of work piece holders 22 is not covered.

Referring back to FIG. 1, a plurality of work pieces 40 is placed intowork piece holders 22, with each of work piece holders 22 holding onework piece 40. In an embodiment, work pieces 40 are package substratesor interposers, which do not have active devices such as transistorstherein. In alternative embodiments, work pieces 40 are device dieshaving active devices, such as transistors (not shown) therein.Throughout the description, work pieces 40 are alternatively referred toas (package) substrates 40 although they can also be other types of workpieces. Metal bumps 42 are formed on the top surfaces of substrates 40.Length L3 and width W3 of substrates 40 are smaller than the respectivelength L1 and width W1 of work piece holders 22. Furthermore, at leastone, and possibly both, of length L3 and width W3 of substrates 40 aregreater than the respective length L2 and W2 of openings 24 of cover20B.

Referring to FIG. 3, die-attach head 28 is used to pick up work pieces30, which may also be device dies, interposers, package substrates, orthe like. Throughout the description, work pieces 30 are also referredto as dies 30, although they can also be other types of work pieces.Dies 30 may be sawed from a wafer as schematically illustrated as wafer32, although dies 30 may be placed in a die tray (not shown), and hencedie-attach head 28 picks up dies 30 from the die tray. Die-attach head28 is configured to use a vacuum to pick up die 30, and to dip solderbumps 34 (not shown in FIG. 3, please refer to FIG. 4) that are on thesurfaces of dies 30 in flux. Die-attach head 28 is a part of a toolmodule (referred to as a die-attach module hereinafter) for performingthe die-attaching process.

FIG. 4 illustrates the die-attaching of die 30 onto substrate 40 usingcompressive placement, which is also performed using die-attach head 28.During the die-attaching, body 20A of jig-type substrate carrier 20 maybe placed on, and possibly secured onto, platform 45, for example, usinga vacuum. Platform 45 is also a part of the die-attach module. After thedie-attaching, solder bumps 34 are in contact with metal bumps 42 ofsubstrate 40. Although bumps 34 are referred to as solder bumps, theymay also be non-reflowable metal bumps such as copper pillar bumps.However, at least one type of bumps 34 and 42, and possibly both, aresolder bumps. Accordingly, the solder bumps that are located betweendies 30 and substrate 40 are referred to as solder bumps 34/42hereinafter. To finish the die-attaching, die-attach head 28 applies adownward force, for example, less than about 5 NT, so that solder bumps34 may stick to metal bumps 42. Die-attach head 28 may repeat theprocess of picking, flux-dipping, and compressive placement of dies 30until all of substrates 40 have dies 30 placed thereon.

During the die-attaching, die-attach head 28 may be at a lowtemperature, which is lower than about 50° C., for example, and may beat room temperature. After the die-attaching of dies 30, cover 20B isplaced on and secured on body 20A (refer to FIG. 5B), and hence jig-typesubstrate carrier 20, substrates 40, and dies 30 form an integratedpart, and may be transferred away from the die-attach module, so thatsolder bumps 34/42 may be reflowed. It is observed that the horizontallength L4 and width W4 (W4 is not shown in FIG. 4, please refer to FIG.10) of dies 30 are smaller than the respective dimensions L2 and W2 ofopenings 24 in FIG. 2. Accordingly, cover 20B may be placed on body 20Aeven if dies 30 have already been attached onto substrates 40.

FIGS. 5A and 5B illustrate a perspective view and a cross-sectionalview, respectively, of the reflow of solder bumps 34/42. Jig-typesubstrate carrier 20 may be placed on platform 48 (FIG. 5B) that cansecure jig-type substrate carrier 20 using a vacuum. Further, platform48 may pre-heat substrate 40 before the subsequent reflow process, forexample, to a temperature lower than 100° C. Next, as shown in FIG. 5A,a plurality of heating heads 46, which are parts of multi-head heatingtool 44, contact the top surfaces of dies 30, with each of heating heads46 contacting one of dies 30. As shown in FIG. 5B, heating heads 46 heatdies 30 until solder bumps 34/42 are melted.

Next, as shown in FIG. 6, heating heads 46, which also have the abilityto pickup dies 30 using a vacuum, lift up dies 30, as symbolized byarrows 47. With the surface tension of the melted solder bumps 34/42,substrates 40 are also lifted up. During the lifting process, substrates40 hang freely under the respective dies 30, and are free to move alonghorizontal directions, as symbolized by arrows 49. Accordingly,substrates 40 are self-aligned to the respective overlying dies 30.Since length L3 and/or width W3 (FIG. 1) of substrates 40 are greaterthan the respective length L2 and/or width W2 of openings 24 (FIG. 2),substrate 40 are blocked by portions of cover 20B, which portions arereferred to as blocking arms 20W hereinafter. Blocking arms 20W aredirectly over and overlap edge portions of the respective substrates 40.The lift-up distance of dies 30 is adjusted so that after substrates 40are blocked by cover 20B, dies 30 may continued to be lifted upslightly, and the melted solder bumps 34/42 may be stretched in thevertical direction. As a result, the height of the melted solder bumps34/42 is adjusted, and the likelihood of bridging between neighboringsolder bumps 34/42 is reduced. The temperature of heating heads 46 maythen be reduced to below the melting temperature of solder bumps 34/42,and hence the melted solder bumps 34/42 solidify. Heating heads 46 thenrelease the respective dies 30.

FIGS. 7 through 9 illustrate cross-sectional views of intermediatestages in the reflow of solder bumps between dies and package substratesin accordance with alternative embodiments. Unless specified otherwise,the reference numerals in these embodiments represent like elements inthe embodiments illustrated in FIGS. 1 through 6. The initial steps ofthese embodiments are essentially the same as shown in FIGS. 1 through5A. FIG. 7 illustrates a cross-sectional view of a portion of thestructure after cover 20B has been placed on body 20A of jig-typesubstrate carrier 20. The thickness of body 20A of jig-type substratecarrier 20 is selected so that the bottom of blocking arm 20W contactedge portions of the top surface of substrates 40, and hence substrates40 are fixed in position, and is not able to move vertically andhorizontally. Platform 48 may be used to pre-heat substrate 40 beforethe subsequent reflow process, for example, to a temperature lower than100° C.

The plurality of heating heads 46 of multi-head heating tool 44contacts, the top surfaces of dies 30, with each of heating heads 46contacting one of dies 30. Heating heads 46 heat dies, so that solderbumps 34/42 are melted.

Next, referring to FIG. 8, heating heads 46 release the respectiveunderlying dies 30, for example, by releasing vacuum. Further, heatingheads 46 may be lifted up, and are not in contact with dies 30 for ashort period of time. In an exemplary embodiment, this period of time isbetween about 1 second and 2 seconds. In other embodiments, this periodof time is between about 0.5 second and about 4 seconds. During thisperiod of time, substrates 40 are still secured in position by blockingarms 20B′. However, since solder bumps 34/42 are in a liquid state, dies30 are free to move slightly horizontally and vertically. Accordingly,with the ability to move in the horizontal directions, dies 30 areself-aligned with the respective underlying substrates 40.

Next, as shown in FIG. 9, heating heads 46 are placed in contact withdies 30 again, and then lift up dies 30 slightly, as also symbolized byarrows 47. In these embodiments, substrates 40 are not allowed to movefreely along horizontal and/or vertical directions due to blocking arms20B′ that hold substrates 40 in position. The lift-up distance of dies30 is adjusted so that the height of the melted solder bumps 34/42 isadjusted to a desirable value, and the likelihood of bridging betweenneighboring solder bumps 34/42 is reduced. The temperature of heatingheads 46 may then be reduced to below the melting temperature of solderbumps 34/42, and hence the melted solder bumps 34/42 solidify. Heatingheads 46 then release the respective dies 30.

In an embodiment, the reflow is performed by a reflow module, whichincludes multi-head heating tool 44 and platform 48. In an embodiment,the die-attach module for performing the die-attaching (FIG. 4) and thereflow module for performing the reflow (FIGS. 5A through 9) areseparate tool modules belonging to a same TCB tool. Alternatively, thedie-attach module and the reflow module belong to separate tools.

FIGS. 10 and 11 illustrate two reflow processes performed usingmulti-head heating tool 44. Referring to FIG. 10, multi-head heatingtool 44 includes two heating heads 46, and hence may perform bonding fortwo dies 30 at a time. After the bonding of two dies 30 is finished,multi-head heating tool 44 may move to the next two dies 30 to performreflow. Arrows 50 illustrate a likely path on which multi-head heatingtool 44 moves. In FIG. 11, multi-head heating tool 44 has four heatingheads 46, and hence may perform bonding for four dies 30 at a time.After the bonding of four dies 30 is finished, multi-head heating tool44 may move to the next four dies 30. Similarly, arrow 50 illustrates alikely path on which multi-head heating tool 44 moves. In the exemplaryembodiments as shown in FIGS. 10 and 11, the total number of work pieceholders in jig-type substrate carrier 20 is 32. Accordingly, the numberof heating heads 46 in multi-head heating tool 44 may be as great as 32,and as small as 2, or may be equal to any other applicable number, suchas 2, 4, 8, 12, and 16.

Since heating heads 46 (FIGS. 5A and 5B) do not perform the task ofpicking and placing dies 30, the temperature of heating heads 46 may bemaintained at high temperatures. For example, FIG. 12 schematicallyillustrates a temperature profile of heating heads 46. In the beginningof a first reflow process, heating heads 46 are at temperature T1, whichis higher than room temperature, and may be higher than about 150° C.,or even greater than about 180° C. Temperature T1 is also lower than themelting temperature T0 of solder bumps 34/42, which may be about 220° C.to about 260° C., for example. During the first reflow process, thetemperature of heating heads 46 ramps up to temperature T2, which ishigher than temperature T0 and is high enough to cause the melting ofsolder bumps 34/42. After the reflow process, the temperature of heatingheads 46 is lowered, for example, back to temperature T1, or even lower.The temperature profile in the first reflow may be repeated for thesecond reflow process and additional reflow processes. Maintainingheating heads 46 at high temperatures may reduce the temperatures'ramp-up time and cool-down time, and hence can also results in theimprovement in the throughput of the TCB process. In alternativeembodiments, after the first reflow process, the temperature of heatingheads 46 may also return back to a low temperature, for example, closeto room temperature.

By separating the die-attaching and the reflow processes to two separatetool modules, the throughput of the TCB bonding may be improved.Further, the use of a multi-head heating tool results in furtherimprovement in the throughput. The jig-type substrate carrier makes thehorizontal movement of substrates possible, and with the self-alignment,the accuracy of the alignment between dies and substrates is improved.

In accordance with embodiments, a method of bonding includes providing afirst work piece, and attaching a second work piece on the first workpiece, with a solder bump disposed between the first and the second workpieces. The second work piece is heated using a heating head of aheating tool to melt the solder bump. After the step of heating thesecond work piece, one of the first and the second work pieces isallowed to move freely in a horizontal direction to self-align the firstand the second work pieces. After the step of allowing one of the firstand the second work pieces to move, a temperature of the heating head islowed until the first solder bump solidifies to form a second solderbump.

In accordance with other embodiments, a method includes providing ajig-type substrate carrier including a plurality of work piece holders;placing a plurality of first work pieces into the plurality of workpiece holders, wherein edge portions of the plurality of first workpieces are directly under and vertically overlap blocking arms of thejig-type substrate carrier; and placing a plurality of second workpieces over the plurality of first work pieces, with solder bumpsjoining the plurality of second work pieces to the plurality of firstwork pieces. The method further includes reflowing the solder bumps.Further, after the solder bumps melt, the plurality of second workpieces is lifted up with the solder bumps in a melting state, whereinthe plurality of first work pieces is lifted up to hang under theplurality of second work pieces, and wherein the jig-type substratecarrier is not lifted up. A temperature of the solder bumps is thenlowered to below a melting temperature of the solder bumps.

In accordance with yet other embodiments, a method includes providing abody of a jig-type substrate carrier comprising a plurality of workpiece holders; placing a plurality of first work pieces into theplurality of work piece holders; attaching a plurality of second workpieces onto the plurality of first work pieces, with solder bumpsjoining the plurality of second work pieces to the plurality of firstwork pieces; placing a cover of the jig-type substrate carrier onto thebody of the jig-type substrate carrier, wherein the cover comprisesblocking arms extending directly over, and contacting, edges of theplurality of first work pieces; placing a plurality of heating headsover, and reflowing, the solder bumps, wherein each of the plurality ofheating heads contacts one of the plurality of second work pieces;reflowing the solder bumps by using the plurality of heating heads toheat the solder bumps; after the solder bumps melt, releasing theplurality of second work pieces from the plurality of heating heads; andafter the step of releasing, lifting up the plurality of second workpieces using the plurality of heating heads, with the solder bumps in amelted state

In accordance with yet other embodiments, a method includes providing abody of a jig-type substrate carrier comprising a plurality of workpiece holders; placing a plurality of first work pieces into theplurality of work piece holders; attaching a plurality of second workpieces onto the plurality of first work pieces using a first toolmodule, with solder bumps joining the plurality of second work pieces tothe plurality of first work pieces; placing a cover of the jig-typesubstrate carrier onto the body of the jig-type substrate carrier;transferring the jig-type substrate carrier and the plurality of firstand the second work pieces to a second tool module; and reflowing thesolder bumps using a plurality of heating heads of a multi-head heatingtool, with the plurality of heating heads contacting the plurality ofsecond work pieces. The multi-head heating tool is comprised in thesecond tool module.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

1. An apparatus for bonding a plurality of dies, the apparatuscomprising: a multi-head heating tool comprising a plurality of heatingheads configured to heat the plurality of dies simultaneously to atemperature higher than a melting temperature of solder regions of theplurality of dies.
 2. The apparatus of claim 1, wherein the plurality ofheating heads is configured to pick up the plurality of dies throughvacuuming.
 3. The apparatus of claim 2, wherein the plurality of heatingheads is configured to heat and to melt solder regions at a same time ofthe vacuuming.
 4. The apparatus of claim 2, wherein the plurality ofheating heads is configured to release the vacuum.
 5. The apparatus ofclaim 1, wherein the plurality of heating heads is arranged as an array.6. The apparatus of claim 1 further comprising a jig-type substratecarrier comprising a body, wherein the body comprises a plurality ofwork piece holders configured to hold dies, and wherein the plurality ofheating heads is configured to be aligned to the plurality of work pieceholders with a one-to-one correspondence.
 7. The apparatus of claim 6,wherein the jig-type substrate carrier further comprises a covercomprising a plurality of through openings, wherein the plurality ofthrough openings is configured to be aligned to the plurality of workpiece holders with a one-to-one correspondence, and wherein the cover isconfigured so that when the cover is placed on and aligned to the body,edge portions of each of the plurality of work piece holders are coveredby the cover, and a center portion of each of the plurality of workpiece holders is exposed through a corresponding one of the plurality ofthrough openings.
 8. The apparatus of claim 1, wherein the plurality ofwork piece holders is arranged as an array.
 9. An apparatus comprising:a multi-head heating tool comprising a plurality of heating headsconfigured to perform heating; and a jig-type substrate carriercomprising a body comprising a plurality of recesses, wherein theplurality of recesses is configured to be aligned to the plurality ofheating heads with a one-to-one correspondence.
 10. The apparatus ofclaim 9, wherein the jig-type substrate carrier further comprises acover comprising a plurality of through openings, and wherein theplurality of through openings is configured to be aligned to theplurality of recesses with a one-to-one correspondence.
 11. Theapparatus of claim 10, wherein sizes of the plurality of throughopenings are smaller than respective sizes of the plurality of recesses.12. The apparatus of claim 10, wherein the cover is configured so thatwhen the cover is placed on the body, edge portions of each of theplurality of recesses are covered by the cover, and a center portion ofeach of the plurality of recesses is exposed through a corresponding oneof the plurality of through openings in the cover.
 13. The apparatus ofclaim 9, wherein the plurality of heating heads is arranged as an array.14. The apparatus of claim 9, wherein the plurality of heating heads isconfigured to: picking up a plurality of dies; and heating the pluralityof dies to a temperature high enough so that solder regions of theplurality of dies are molten.
 15. An apparatus comprising: a pluralityof heating heads arranged as an array, wherein the plurality of heatingheads is configured to heat to temperatures higher than meltingtemperatures of solder regions, and wherein the heating heads isconfigured to perform vacuuming.
 16. The apparatus of claim 15, whereinthe plurality of heating heads is configured to perform the vacuumingand heating simultaneously.
 17. The apparatus of claim 15, wherein theplurality of heating heads is configured to release the vacuum.
 18. Theapparatus of claim 15, wherein the plurality of heating heads isarranged as an array.
 19. The apparatus of claim 18, wherein the arrayhas a size equal to or greater than two by three.
 20. The apparatus ofclaim 15, wherein the plurality of heating heads has a count greaterthan 16.